The present invention relates to a method and apparatus for heating a thermal head of a thermal printer, and particularly for accurately heating the thermal head to a target temperature.
A thermal printer which records data such as letters can be classified into a thermo-sensitive and thermo-ink-transfer recording systems. The former realizes recording by changing the color of the recording paper through heating of a heat generating body of a thermal head while the thermal head is directly in contact with said recording paper, which changes color when it is heated, while the latter realizes recording by heating the ink with the thermal head to dissolve or vaporize it for transferring the ink to the recording paper.
FIG. 1 shows the outline of a thermo-transfer recording unit of a line recording system. FIG. 2 shows the structure of the thermal head. The thermal head 11 faces a platen 14 through an ink sheet 12 and a recording paper 13. The ink of the ink sheet 12 melts when it is heated by the thermal head 11, and it is transferred to a recording paper 13 for the recording. The thermal head 11 has a heat generating body for each single line arranged along the direction perpendicular to the paper surface, and recording of each single line is carried out almost simultaneously. Upon completion of recording of a single line, the recording paper 13 and ink sheet 12 are transferred simultaneously in the direction indicated by the arrow.
The thermal head 11 has the multi-layer structure as shown in FIG. 2. Namely, a glaze layer 24, a heat generating body 23 and an electrode 22 are provided in a layer structure on a substrate 25, and a protection layer 21 is provided at the surface in contact with the recording paper. 26 is a heat sink.
When multi-level recording is to be carried out using such a thermal head, a particular problem arises, which cannot be foreseen in the existing binary level recording. Explanation is made hereunder for this problem in order to understand the background of the present invention more easily.
Namely, the problem is that the allowable range of heating temperature for temperature control of a thermal head is narrow, the temperature control must be done accurately, and it is difficult to realize accurate temperature control.
FIG. 3 is given for convenience in explaining such a problem. Here, the term binary level recording refers to the recording mode where recording is conducted in black or white by the thermal head, while the term multi-level recording refers to the recording mode wherein recording is conducted with densities of intermediate tone in accordance with the recording temperature.
FIG. 3 shows the relation between temperature and recording density, wherein the horizontal coordinate plots target temperature T while the vertical coordinate plots recording density D. In thermal recording, recording density corresponds to time of applying the temperature. However, for a brief explanation, the recording density is explained as corresponding only to the level of the temperature in FIG. 3.
In the temperature control for binary level recording, it is sufficient to control the recording to be black or not. It is sufficient to respectively control the temperature within the allowable temperature region .DELTA.T.sub.1 for the white area, and within the allowable temperature region .DELTA.T.sub.2 for the black area of the recording medium.
In this case, both .DELTA.T.sub.1 and .DELTA.T.sub.2 are widths of the temperature range and it is enough to control the heating temperature of the heat generating element of the thermal head so that it is restricted to such widths.
On the other hand, in the case of three-level recording in the multi-level recording temperature control, if it is required to obtain the recording densities D.sub.a, D.sub.b and D.sub.c, the temperature must be set respectively up to the target temperatures T.sub.a, T.sub.b, T.sub.c, and the widths of allowable temperature .DELTA.T.sub.a, .DELTA.T.sub.b, .DELTA.T.sub.c become very narrow. The multi-level recording requires controlling the temperature in the narrow allowable ranges. Examples of actual allowable temperature ranges are indicated below.
In binary level recording by a thermal printer in which the line period is 5 msec and the time for pressing the thermal head to the recording paper after it is heated to the specified temperature is 1 msec, T.sub.1 is set to 80.degree. C. (20.degree. C. 100.degree. C.) and T.sub.2 is set to 150.degree. C. (150.degree. C. 300.degree. C.).
On the other hand, in the multi-level recording by the thermal printer where the line period is set to 5 msec, and the time for pressing the thermal head to the recording paper after it is heated to the specified heating temperature is set to 1 msec, the maximum allowable temperature range is 3.degree. C. in case the temperature range from 100.degree. C. to 150.degree. C. is divided into 16 tones (16 levels).
The problem in the temperature control for multi-level recording is explained in detail with reference to FIG. 4.
In FIG. 4, the vertical coordinate indicates temperature T, while the horizontal coordinate indicates time t. The time charts indicated by waveforms (a), (b) and (c) show the drive signal for heating to be applied to a heat generating element of the thermal head.
It is assumed that the thermal recording is carried out in the period from time t.sub.0 to time t.sub.1.
The temperature of the heat generating element rises as indicated by a curve shown in FIG. 4.
When the drive pulses P.sub.a, P.sub.b, P.sub.c having the widths W.sub.a, W.sub.b, W.sub.c indicated by waveforms (a), (b), and (c) in FIG. 4 are applied to the heat generating element, the heat generating element is heated up to the temperature A, B, and C at the time t.sub.a, t.sub.b, t.sub.c.
The densities corresponding to the temperatures A, B, and C can be obtained as the recording densities.
The heat generating element is heated to any of the temperatures T.sub.a, T.sub.b, T.sub.c (T.sub.a &gt;T.sub.b &gt;T.sub.c) by the time t.sub.1, namely at the end of the recording period.
Since in the past the stored thermal energy of the heat generating element is different accordance with the drive condition of the drive signal, the temperature of the heat generating material is different at the end of the recording period.
Namely, at the time of starting the drive signal for the next heating, a temperature difference already exists due to the difference of stored thermal energy.
When temperature differences at the time of starting the drive signal are generated as explained above, it is very difficult to accurately control the temperature up to the target heating temperature.
In such a case, it was attempted to control the power application of the next recording pulse signal in accordance with the temperature history, namely, the past temperature of the heat generating element for recording, in order to accurately control the temperature up to the target heating temperature. This is referred to as the temperature history system. In the case of binary recording by this system, if the preceeding recording is black, the pulse width of the drive signals to be applied to the heat generating element is more curtailed than in the case of white, or the pulse amplitude is made smaller and thereby the supplied power is reduced.
As explained above, the change of recording density due to stored heat of the thermal head can be reduced by employing the temperature history system, but this system has the following disadvantage. In the temperature history system, it is essential to obtain the desired temperature of the heat generating element at the starting time of each successive period recording by applying the past drive conditions to the theoretical equations for the heating and cooling characteristics, and to obtain the condition of the drive signal corresponding to the amount of heat to be supplied from such temperature. This calculation is very complicated, for instance it includes an exponential function, and it is necessary to apply such calculations to all the heat generating elements provided to the thermal head (for example, when the recording paper is size A4 with 8 dots/mm, 1680 heat generating elements are necessary), and a circuit for realizing high speed calculation is also required.
The more the temperature history in the past is relied on, the more the error of recording density due to the stored heat is reduced, however, it costs more time to calculate a longer history.
Moreover, the recording period of a heat generating element is not constant, because the recording is conducted at a high speed and the instantaneous power consumption is limited. Consideration must be taken for change in the recording period in order to calculate the conditions of the drive signal from the drive conditions in the past. The calculation is complicated and the drive conditions are remarkably diverse.
In the following example, the temperature controls for binary recording and multi-level recording are compared with reference to the calculation time and the amount of calculations.
For calculation of history from the preceeding five time slots, 2.sup.5 calculations are necessary in the case of binary control, but almost 10.sup.6 calculations (16.sup.5) are necessary for the multi-level control of 16 levels which is an object of the present invention. Namely, the amount of calculations for multi-level control is about 30,000 times the binary level control.
This means that a significant calculation time is required when the thermal recording speed is considered.
Namely, since the next drive signal cannot be applied before completion of calculation of the thermal history, the recording period is substantially lowered. Moreover, a high speed and high precision operation circuit is required.